Add more aggregations and operations to PX Query Builder report

Our Product team PX users would like to have more flexibility in how they slice and dice data. One way that seems logical to do that is using the Query Builder report. However, it does not allow aggregations other that total, so using it to find the average number of events for a period, for instance, isn’t possible. 

This arose when one of the team asked how to build a report that would show the average number of events/day. There is a an average per sessions widget, so it could also be helpful to add different period options there.